A solution culture experiment was conducted to study effects of NH₄⁺-N/NO₃^--N ratio and phosphorus level on growth,and nitrogen uptake of spinach and activities of nitrate reductase (NRA),glutamine synthetase (GSA) therein.Results show that spinach plants in nutrient solutions,the same in phosphorus supply level,decreased in biomass with decreasing NH₄⁺-N/NO₃^--N ratio,but differed slightly between solutions of 25 75 and 0 100 in NH₄⁺-N/NO₃^--N ratio.In treatments the same in NH₄⁺-N/NO₃^--N ratio,spinach plants increased in biomass with rising phosphorus level in the nutrient solutions.Nitrate levels in spinach shoots increased with decreasing NH₄⁺-N/NO₃^--N ratio and phosphorus level,but did not vary much between treatments different in NH₄⁺-N/NO₃^--N ratio.Spinach nitrogen content increased with phosphorus level,but did not show much difference between treatments different in NH₄⁺-N/NO₃^--N ratio.Nitrogen accumulation in spinach markedly increased with decreasing NH₄⁺-N/NO₃^--N ratio and increasing phosphorus level,which were concurrent with the increase in spinach biomass.In solutions of mixed NH₄⁺-N and NO₃^--N,phosphorus deficiency significantly inhibited spinach uptake of NH₄⁺-N and NO₃^--N.When the NH₄⁺-N/NO₃^--N ratio was 25 100,the NH₄⁺-N amount absorbed by spinach growing in the solution deficient in phosphorus ac counted for 27.2% of the total nitrogen uptake,however,it dropped to 16.9% in the solution adequate in phosphorus.When the NH₄⁺-N/NO₃^--N ratio was kept at 50 50,the value was 45.4% and 23.9%,respectively.Apparently,the uptake of NH₄⁺-N was less depressed by phosphorus deficiency than the uptake of NO₃^--N.With increasing phosphorus level,NRA of spinach markedly increased in treatments the same in NH₄⁺-N/NO₃^--N ratio,but with higher NH₄⁺-N/NO₃^--N ratios,NRA of spinach was depressed,and it increased when the ratio was lower.Moreover,phosphorus deficiency depressed GSA seriously.At the same phosphorus level,GSA of spinach shoot increased pronouncedly with increasing NH₄⁺ N in nutrient solutions.There fore,it is feasible to decrease nitrate level of some leafy vegetables that are grown on those soils high in nitrate by applying more phosphorus fertilizer.